Petrov et al., Journal of General Chemistry of the USSR (English Translation), Vol. 26, No. 1, pp. 49-51 (January, 1956) teach the reaction between 4-vinylpyridine and a diene, such as butadiene or isoprene, to form a Diels-Alder product.
Copending applications Ser. No. 300,046, filed Sept. 8, 1981, in the name of Thomas J. Walter (Walter I), now U.S. Pat. No. 4,405,792 and Ser. No. 497,026, filed May 23, 1983, in the name of V. Ramachandran (Ramachandran I), disclose Diels-Alder reactions between haloprenes and 4-vinylpyridines, preferably in the presence of a boron trifluoride catalyst, to form 4-(halocyclohex-3-enyl)pyridines which are mixtures of 4-(4-halocyclohex-3-enyl)pyridine and 4-(3-halocyclohex-3-enyl)pyridine isomers in a mol ratio of about 65:35.
Since:
(1) an objective of Walter I and Ramachandran I is to form 4-(halocyclohex-3-enyl)pyridines useful in the synthesis of the antibacterial 1-alkyl-1,4-dihydro-4-oxo-7-pyridinyl-3-quinolinecarboxylic acids of Sterling Drug's U.S. Pat. Nos. 3,753,993 (Lesher et al.), 3,907,808 (Lesher and Carabateas), and 4,118,557 (Lesher),
(2) 4-(4-halocyclohex-3-enyl)pyridines are the 4-(halocyclohex-3-enyl)pyridine isomers that are useful in this regard, and
(3) the inherent formation of a substantial amount of the 4-(3-halocyclohex-3-enyl)pyridine isomers in their processes reduces the yield of 4-(4-halocyclohex-3-enyl)pyridine and derivatives that might otherwise be obtained in the practice of their inventions,
it would be advantageous to modify their processes to provide a more favorable isomer distribution. It would be particularly advantageous to provid a Diels-Alder reaction that would not only lead to this more favorable isomer distribution in the preparation of 4-(halocyclohex-3-enyl)pyridines but also be useful in the preparation of other adducts.